1. Field
The present invention relates generally to telecommunications, and, more specifically, to frequency domain equalization in high speed data packet access (HSDPA) point-to-multipoint communication systems.
2. Background
A modern communication system is expected to provide reliable data transmission for a variety of applications, including voice and data applications. In a point-to-multipoint communications context, known communication systems are based on frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and perhaps other multiple access communication schemes.
A CDMA system may be designed to support one or more CDMA standards, such as (1) the “TIA/EIA-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (this standard with its enhanced revisions A and B will be referred to as the “IS-95 standard”), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the “IS-98 standard”), (3) the standard sponsored by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including documents known as 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the “W-CDMA standard”), (4) the standard sponsored by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” (the “cdma2000 standard” collectively), (5) the 1xEV-DO standard “TIA/EIA/IS-856 cdma2000 High Rate Packet Data Air Interface Specification”, and (6) certain other standards. The standards listed above are incorporated by reference as if fully set forth herein, including annexes, appendices, and other attachments.
In 3GPP Release 5 HSDPA, a base transceiver station of an access network sends downlink payload data to user equipment devices on High Speed Downlink Shared Channel (HS-DSCH), and the control information associated with the downlink data on High Speed Shared Control Channel (HS-SCCH). There are 256 Orthogonal Variable Spreading Factor (OVSF or Walsh) codes used for data transmission. In HSDPA systems, these codes are partitioned into release 1999 (legacy system) codes that are typically used for cellular telephony (voice), and HSDPA codes that are used for data services. In the OVSF code tree structure, the partitioning is such that if a node belongs to the HSDPA, then all sub-nodes of that node also belong to HSDPA. In known systems, codes are partitioned at a level where there are—16 nodes, with one node assigned to other than HSDPA communications. Because zero code is not usable, a maximum of 15 codes can be used for HSDPA. For HSDPA transmissions, the code allocation is dynamic, and can generally change from one transmission time interval (“TTI”) to the next. Thus, there can be more than 15 HSDPA devices served within a cell. The more HSDPA codes a device is allocated, the higher the downlink data rate available to that device.
For each TTI, the dedicated control information sent to an HSDPA-enabled user equipment device indicates to the device which codes within the code space will be used to send downlink payload data to the device, and the modulation that will be used for transmission of the downlink payload data. The control information is sent together with its associated cyclic redundancy check (CRC) block. The CRC block is obtained by computing a 16-bit CRC for the control information packet, and then performing an exclusive OR (XOR) operation on the computed CRC and the unique 16-bit ID of the user equipment device. The 16-bit CRC of each control channel is thus masked with the unique 16-bit ID of the user equipment device. Each user equipment device monitors a number of control channels, four, for example, but receives the dedicated control information intended for the device on only one control channel specific to the particular user equipment device. The user equipment device does not evaluate CRCs for any of the other user equipment devices in the cell, and does not have the control data for the other user equipment devices.
It is desirable to cancel out interference from the downlink data received by the user equipment devices. Interference can be classified as inter-symbol interference (ISI), which is from the same user due to multi-paths, and inter-user interference (IUI), which is interference from other users. Most of the interference received from the serving HSDPA cell/site is inter-symbol interference because the codes are orthogonal. But the orthogonality assumption holds generally only for additive white Gaussian noise (AWGN) transmission channels. If there is multi-path, code orthogonality is lost to some degree, and inter-user interference arises. In known code division multiple access systems, performance degradation due to inter-user interference is simply tolerated. It would be desirable, however, to cancel out inter-user interference to improve system performance.
Interference cancellation is carried out by equalization. Knowledge of modulation techniques used within the code space is necessary for effective equalization of received signals. Release 1999 codes are generally used with Quadrature Phase-Shift Keying (QPSK), so user equipment devices have this information. But as noted above, in HSDPA systems both modulation and code allocation are user-specific and dynamic, and a specific user equipment device does not have this information for HSDPA codes other than the device's own allocated codes. Because of higher data rates envisioned for HSDPA systems, inter-user interference becomes more significant.
Interference cancellation may also be carried out by the user equipment device by subtracting known signals received from HSDPA cells/sites other than its serving cell/site. To do this more efficiently, the user equipment device needs to have information about the HSDPA codes allocated in those other cells/sites.
There is therefore a need in the art for methods and apparatus that convey to the user equipment devices information regarding code allocation of HSDPA codes and modulation used with the HSDPA codes. There is also a need in the art to convey such information within the framework of legacy CDMA systems and with minimal consumption of bandwidth, power, and computational resources.